The polymerization of olefins, particularly propylene, is well known and well described in the patents and literature. A number of reaction, catalyst deactivation and product recovery schemes are also well known and widely described. However, certain methods for each step are known to have considerable advantages over other methods such as, for example, an autorefrigerated reaction vessel wherein the monomer, diluent if applicable, catalyst and, when applicable, molecular hydrogen are brought together in a reaction zone having both a liquid and vapor phase. One such reaction scheme utilizing an autorefrigerated reactor is described in U.S. Pat. Nos. 3,259,614 and 4,058,652, for example. The vaporization of the liquid, preferably propylene, in which the reaction is being conducted, removes heat from the reaction mixture, thus maintaining the temperature and pressure relationships in the reaction medium. The propylene thus removed can then be cooled either by indirect heat exchange or direct contact with cooler fluids, usually liquid propylene, and returned to the reactor. Often this return of the liquid propylene provides a convenient way for adding catalyst components, or hydrogen, to the reaction zone. The autorefrigerated reaction scheme is advantageous for use in the manufacture of polypropylene since heat removal is simplified and more efficient allowing the advantages of large scale to be realized. Fouling problems are minimized resulting in longer run length before cleaning.
Further, in the recovery of solid polymeric propylene or copolymers of propylene and other olefins such as, for example, ethylene, it is known to clean the solid polymer containing catalyst residue by a liquid phase washing with a hydrocarbon, such as heptane and/or alcohol, to remove the catalyst residues, or "ash," and amorphous polymer to provide a purified product which will meet commercial product specifications. To meet such specifications, the catalyst residue and amorphous polymer must be removed to the extent that the polypropylene contain not more than about 5 to 15 ppm titanium, about 10 ppm aluminum, and less than about 20 ppm of chlorine. Typically, purified polypropylene product would contain at least about 95% boiling heptane insoluble isotactic polypropylene. However, hydrocarbon/alcohol wash methods require considerable separation steps and energy consumption to recover and recycle the materials.
Another known washing technique, washing with liquid propylene, is particularly preferred since it avoids the introduction of extraneous hydrocarbons (diluent), alcohols, etc. that require additional separation steps to prevent their introduction and/or other resulting polar compounds into the polymerization reactor (during recycle). Washing with liquid propylene is satisfactory for lowering the impurities content such as catalyst residues and amorphous, soluble, polymer to acceptable levels. The liquid propylene wash of solid propylene polymers is described in many U.S. patents such as, for example, U.S. Pat. No. 3,356,669 which describes a liquid propylene wash system as well as other prior art extractants for catalyst residues. But such patent points out the difficulty of using such system heretofore in that it requires the use of a liquid phase polymerization reaction zone and catalyst deactivation system to take advantage of the liquid phase wash with propylene. Moreover, the liquid propylene wash system described therein is operated batch-wise, utilizing progressively decreasing temperatures for the liquid propylene wash media.
U.S. Pat. No. 3,259,614 describes a process whereby the propylene is polymerized in an autorefrigerated reactor. The patent further discloses introducing propylene into the bottom of a draw-off leg to wash propylene soluble catalyst residue and propylene soluble polymer countercurrently back into the bottom of the reactor. However, even with this procedure the propylene insoluble catalyst must still be deactivated, solubilized and removed from the polymer slurry in order to produce purified solid polymer product. In the described process, such would require the use of a deashing zone separate from the polymerization reactor and draw-off leg to avoid the introduction of catalyst deactivating agent into the reactor which otherwise would have adverse effects on the polymerization reaction.
Belgium Pat. No. 824,438, published May 15, 1975, also describes a system whereby a liquid phase reactor using propylene as the reaction medium is used in conjunction with a counter-current liquid propylene wash column. Considerable accommodation is necessary in order to provide cooling for the reactor. Moreover, there is no mention of the problem of flashing of the polymer slurry at the entrance into the wash zone as described hereafter. An autorefrigerated reactor which has a propylene liquid phase and a vapor phase system could have helped solve the cooling problem but, until now, a method for combining an autorefrigerated reactor and a liquid propylene wash was not available.
Thus, it is seen from the foregoing and other patents in the art that washing polymer impurities such as, for example, catalyst residues and soluble polymer, from solid propylene polymers by liquid propylene washing and preparing a solid propylene polymer in an autorefrigerated reactor are, separately, known. Yet, until this invention, those skilled in the art have not heretofore been able to take advantage of these two individually desirable processing steps in a single method. Direct introduction of the reactor slurry into the liquid propylene wash zone with the attendant pressure drop necessary to convey the polymer slurry would cause flashing of propylene to vapor with the resulting agitation adversely affecting the efficient operation of the product wash. The following described invention combines these two highly desirable processing steps into an efficient unitary method for the preparation of polypropylene or copolymers of propylene and other olefins, i.e., ethylene.